The present invention relates to methods and apparatus for embossing expanded graphite sheet material under vacuum.
Electrochemical fuel cells convert reactants, namely fuel and oxidants, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. The electrodes both comprise an electrocatalyst disposed at the interface between the electrolyte and the electrodes to induce the desired electrochemical reactions. The fuel fluid stream which is supplied to the anode may be a gas such as, for example, substantially pure hydrogen or a reformate stream comprising hydrogen. Alternatively, a liquid fuel stream such as, for example, aqueous methanol may be used. The oxidant fluid stream, which is supplied to the cathode, typically comprises oxygen, such as substantially pure oxygen, or a dilute oxygen stream such as air.
Solid polymer fuel cells employ a solid polymer electrolyte, otherwise referred to as an ion exchange membrane. The membrane is typically interposed between two electrode layers, forming a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d). While the membrane is typically selectively proton conductive, it also acts as a barrier, isolating the fuel and oxidant streams from each other on opposite sides of the MEA. The MEA is typically disposed between two plates to form a fuel cell assembly. The plates act as current collectors and provide support for the adjacent electrodes. The assembly is typically compressed to facilitate good electrical contact between the plates and the electrodes, and to facilitate adequate sealing between fuel cell components. A plurality of fuel cell assemblies may be combined in series or in parallel to form a fuel cell stack. In a fuel cell stack, a plate may be shared between two adjacent fuel cell assemblies, in which case the plate also serves as a separator to fluidly isolate the fluid streams of the two adjacent fuel cell assemblies.
Fuel cell plates known as fluid flow field plates have open-faced channels formed in one or both opposing major surfaces for directing reactants and/or coolant fluids to specific portions of such major surfaces. The open-faced channels also provide passages for the removal of reaction products, depleted reactant streams, and/or heated coolant streams. For an illustration of a fluid flow field plate, see, for example, U.S. Pat. No. 4,988,583, issued Jan. 29, 1991. Where the major surface of a fluid flow field plate faces an MEA, the open-faced channels typically direct a reactant across substantially all of the electrochemically active area of the adjacent MEA. Where the major surface of a fluid flow field plate faces another fluid flow field plate, the channels formed by their cooperating surfaces may be used for carrying coolant for controlling the temperature of the fuel cell.
Conventional methods of fabricating fluid flow field plates require the engraving or milling of flow channels into the surface of rigid plates formed of graphitized carbon-resin composites. These methods of fabrication place significant restrictions on the minimum achievable cell thickness due to the machining process, plate permeability, and required mechanical properties. Further, such plates are expensive, both in raw material costs and in machining costs. The machining of channels and the like into the graphite plate surfaces causes significant tool wear and requires significant processing times.
Alternatively, fluid flow field plates can be made by a lamination process, as described in U.S. Pat. No. 5,300,370, issued Apr. 5, 1994, wherein an electrically conductive, fluid impermeable separator layer and an electrically conductive stencil layer are consolidated to form at least one open-faced channel. Such laminated fluid flow field assemblies tend to have higher manufacturing costs than single-layer plates, due to the number of manufacturing steps associated with forming and consolidating the separate layers.
Alternatively, fluid flow field plates can be made from an electrically conductive, substantially fluid impermeable material that is sufficiently compressible or moldable so as to permit embossing. Expanded graphite sheet is generally suitable for this purpose because it is relatively impervious to typical fuel cell reactants and coolants and thus is capable of fluidly isolating the fuel, oxidant, and coolant fluid streams from each other; it is also compressible and embossing processes may be used to form channels in one or both major surfaces. For example, U.S. Pat. No. 5,527,363, issued Jun. 18, 1996, describes fluid flow field plates comprising a metal foil or sheet interposed between two expanded graphite sheets having flow channels embossed on a major surface thereof.
However, embossing expanded graphite sheet material can be problematic. During the embossing process, gas (for example, air) that would be advantageously liberated on compression may become trapped within the sheet material, potentially leading to delamination and/or blistering of the embossed material. In some applications, such as fluid flow field plates in a fuel cell, for example, delamination and/or blistering of expanded graphite sheet plates is undesirable. For example, delamination and/or blistering may weaken the plate and may make it more fluid permeable. The plate material is also rendered less homogeneous as a result, and may exhibit undesirable localized differences in conductivity. Delamination and/or blistering can also cause surface defects that may affect the flow channels on the plate. Further, the foregoing problems may be difficult to detect during fabrication and may only surface at a later date. Finally, in applications where the embossed plate is subsequently impregnated with a resin, delamination and/or blistering may result in voids in the plate material that become filled with resin. Where the resin employed is nonconductive, this may result in undesirable nonconductive regions dispersed within the plate.
An improved method of embossing expanded graphite sheet material comprises removing at least a portion of the gas from within the material by exposing the material to a pressure less than atmospheric pressure, and then embossing the material. For example, the material may be embossed in an embossing atmosphere at a reduced pressure less than atmospheric pressure and maintaining a reduced pressure at least during the embossing step. Preferably, the pressure to which the expanded graphite sheet material is exposed is less than or equal to about 400 torr. More preferably, the pressure is less than 350 torr, more preferably less than 170 torr, and more preferably still, less than 50 torr. The embossing atmosphere may comprise an inert gas, such as nitrogen, helium and argon, for example. The method may further comprise continuing to evacuate gases from the embossing atmosphere during the embossing step.
The expanded graphite sheet material may comprise a plurality of sheet materials comprising at least one expanded graphite sheet, and the method may further comprise laminating the plurality of sheet materials during the embossing step. The plurality of sheet materials may comprise at least one sheet of metal foil.
An improved apparatus for embossing expanded graphite sheet material at a pressure less than atmospheric pressure comprises:
(a) at least one embossing device;
(b) at least one compression device adapted to urge the embossing device against the material;
(c) an embossing chamber comprising the at least one embossing device and adapted to receive the material, and to be substantially gas-tight at least when the embossing device is urged against the material by the pressing device; and
(d) an evacuation device for reducing the pressure within the embossing chamber.
The at least one embossing device may consist of plate dies or roller dies, and the at least one pressing device may consist of press platens or rollers. The apparatus may further comprise at least one sealing member associated with the embossing chamber for maintaining the gas-tight seal therein. The sealing member may comprise at least one O-ring, bellows, compressible seal or inflatable bladder. Preferably, the evacuation device is capable of reducing the pressure within the embossing chamber to less than or equal to about 400 torr. More preferably, the evacuation device is capable of reducing the pressure within the embossing chamber to less than 350 torr, more preferably less than 170 torr, and more preferably still, less than 50 torr.
The apparatus may further comprise an inert gas source fluidly connectable to the embossing chamber. The inert gas may be selected from the group consisting of nitrogen, helium and argon.
Another improved apparatus for embossing expanded graphite sheet material comprises:
(a) a chamber adapted to receive the material and to maintain a reduced pressure less than atmospheric pressure at least when the material is in the chamber;
(b) an evacuation mechanism for reducing the pressure within the embossing chamber; and
(c) at least one roller die associated with the chamber for receiving the material from the chamber.
In one embodiment, the chamber of the apparatus comprises at least one resilient member extending from an interior surface of the chamber and adapted to contact the material. For example, the chamber may comprise an upper portion and a lower portion, the upper portion comprising a plurality of resilient sealing members extending downwardly therefrom, and the lower portion comprising a plurality of resilient sealing members extending upwardly therefrom, both sets of sealing members being adapted to contact the material.
The foregoing embodiment may further comprise at least one feed roller for directing the material to the chamber. The feed roller may be a pin roller, for example.
In another embodiment, the chamber of the apparatus comprises at least two rollers adapted to contact a major surface of the material, such that a substantially gas-tight seal is formed between each of the rollers and the surface of the material. The chamber may comprise at least two pair of opposed rollers; each of the pairs of opposed rollers adapted to contact opposing major surfaces of the material. At least one of the rollers may be the roller die(s) associated with the apparatus.
The embodiment of the apparatus may further comprise at least one sealing member associated with the chamber for maintaining a gas-tight seal therein. The sealing member may comprise, for example, a sealing member disposed at each end of the roller(s), a resilient member adapted to contact the cylindrical surface of the roller(s), or both.
The roller(s) may be driven, or the apparatus may further comprise at least one feed roller, such as a pin roller, for directing the material to the chamber.